Field of the Invention
The present invention relates to a wire electric discharge machine.
Description of the Related Art
When electric discharge machining is performed using a wire electric discharge machine, the shape of a machined surface is substantially similar to a transcription of the shape of a wire. Basically, the greater the distance between upper and lower guides, the larger the amount of deflection in the central portion of the wire is. The greater the plate thickness, therefore, the lower the straightness accuracy of the machined surface tends to be.
Methods to solve this problem are described in Japanese Patent Applications Laid-Open Nos. 62-152616, 1-121127, 7-171716, 7-60548, 2004-50298, WO93/01017, WO2007/032114, and WO2008/050404. According to these methods, machining currents flowing through upper and lower conductors are measured by means of an inductor, such as a coil, and a discharge position is calculated based on the ratio between the measured currents. In this way, reduction in shape accuracy, as well as wire breakage, due to discharge concentration, can be prevented. In a similar method described in Japanese Patent Application Laid-Open No. 60-29230, moreover, the same effect is obtained by using a dividing resistor in place of the inductor.
If the plate thickness of a workpiece increases, furthermore, the amount of deflection of the central portion of the wire increases, in particular, so that the amount of a gap (inter-electrode gap) between the wire and the workpiece is reduced. In the prior art, if the machining voltage is increased even momentarily, the state of the inter-electrode gap is determined to be satisfactory, and a high machining current is applied in every electric discharge without regard to a discharge delay time. Consequently, the central portion of the workpiece, in which electric discharge with a short discharge delay time is frequently generated, is excessively machined. Thus, the greater the plate thickness, the lower the straightness accuracy tends to be.
Thereupon, an experiment according to the present invention revealed that the straightness accuracy of a workpiece can be improved by taking advantage of the discharge delay time that is correlated with the inter-electrode gap. Specifically, if the discharge delay time is less than a predetermined value, the amount of machining at the central portion of the plate thickness can be reduced by making the machining current lower than a normal value or by applying no machining current.
Described in Japanese Patent Application Laid-Open No. 58-211826 is a prior art technique that is based on the same discharge delay time as in the present invention, although the object of the prior invention, unlike that of the present invention, is to prevent wire breakage. According to this technique, a current lower than a normal current is supplied to prevent breakage with a narrow inter-electrode gap if the discharge delay time is less than a predetermined value.
Described in Japanese Patent Application Laid-Open No. 10-315052 is a method in which a measured discharge delay time less than a predetermined value is determined to be abnormal and the application of a machining current by a main discharge circuit is interrupted, in order to prevent wire breakage if immediate discharge or the like is generated due to a worsened machining state.
Described in Japanese Patent No. 5510616 is a technique for preventing wire breakage by reducing the time of machining current application, interrupting the application, or inserting a down time if the discharge delay time is less than a predetermined value.
Described in Japanese Patent Applications Laid-Open Nos. 5-177436 and 5-69230 are methods in which the time of machining current application is set in proportion to the discharge delay time in order to prevent breakage due to the machining current application depending on the inter-electrode state.
In the methods described in Japanese Patent Applications Laid-Open Nos. 62-152616, 1-121127, 7-171716, 7-60548, 2004-50298, WO93/01017, WO2007/032114, WO2008/050404, and 60-29230, reduction in shape accuracy, as well as wire breakage, due to discharge concentration, is prevented by detecting the discharge position by means of the coil or the dividing resistor. These methods require the use of a detection circuit composed of the coil or the dividing resistor and an analog circuit for processing analog signals output from the detection circuit. Thus, there is a problem that the configuration is complicated and expensive and a method is needed to correct errors in the detection circuit and the analog circuit.
If the methods described in Japanese Patent Applications Laid-Open Nos. 58-211826, 10-315052, 5-177436, and 5-69230 and Japanese Patent No. 5510616 are used to make an appropriate adjustment for the improvement in straightness accuracy in thick-plate machining, it can be expected that the straightness accuracy in the thick-plate machining can be improved to some extent, in addition to the prevention of wire breakage as a primary object.
If the discharge delay time is zero, that is, in case of a short-circuit state, however, the machining current should not always be reduced. In other words, the discharge delay time becomes zero if the inter-electrode gap is short-circuited by machining chips suspended in the inter-electrode gap so that the machining voltage cannot increase. In such a situation, the machining current should be positively applied to remove the machining chips in the inter-electrode gap. If the machining current is not applied or is reduced, in contrast, the machining chips in the inter-electrode gap cannot be removed, so that the wire and the workpiece are brought into contact with each other, thereby causing a complete short-circuit state. As a result, the relative position of the wire and the workpiece is expected be shifted to widen the inter-electrode stance and cancel the short-circuit state. Since it takes time to cancel the short-circuit state, the machining time is prolonged, and at the worst, no or only a low machining current is applied due to the short-circuit state. Consequently, the short-circuit state cannot be canceled, so that the machining operation is stopped inevitably.